This invention relates to light-polarizing materials and to a method for producing such materials. More particularly, it relates to polyvinyl alcohol light-polarizing materials stabilized against adverse influences of heat and moisture and to a method by which resistance to such influences can be achieved.
Light polarizers, in general, owe their properties of selectively passing radiation vibration along a given electromagnetic vector (and absorbing radiation vibrating along a second given electromagnetic radiation vector) to the anisotropic nature of the transmitting medium. Dichroic polarizers are linear polarizers of the absorptive type and owe their light-polarizing properties to the vectorial anisotropy of their absorption of incident lightwaves. Light entering a dichroic medium encounters two different absorption coefficients, one low and one high. The emerging light vibrates predominantly in the direction of low absorption.
The development of synthetic polarizers has made possible the widespread utility of light-polarizing elements for a variety of applications; and methods for the production of such synthetic polarizers have been known. Among the most widely used synthetic polarizers are the linear dichroic light-polarizing materials of the polyvinyl alcohol type which, in general, comprise a sheet of stretched (oriented) polyvinyl alcohol having light-polarizing (dichroic) molecules, such as polyvinylene molecules formed by heating the oriented polyvinyl alcohol sheet in the presence of a suitable dehydration catalyst such as an atomsphere of hydrogen chloride, or which may be introduced by dyeing the sheet with a dichroic stain such as iodine. By orienting the polyvinyl alcohol matrix unidirectionally, the transition moments of the absorbers are also so oriented and the material becomes visible dichroic.
Methods for the production of light polarizers of the polyvinyl alcohol type are described, for example, in U.S. Pat. No. 2,173,304 (issued Sept. 19, 1939 to E. H. Land et al.); U.S. Pat. No. 2,445,555 (issued July 20, 1948 to F. J. Binda); U.S. Pat. No. Re. 23,297 (reissued Nov. 28, 1950); U.S. Pat. No. 4,166,871 (issued Sept. 4, 1979 to N. W. Schuler); and in U.S. Pat. No. 4,591,512 (issued May 27, 1986 to J. L. Racich et al.). In the manufacture of light polarizer materials, it is known to treat the polarizer material with agents which promote stability of the light polarizing material against, for example, heat, moisture or ultraviolet radiation. Thus, the boration of a polyvinyl alcohol polarizer with an agent such as boric acid is described, for example, in the aforementioned U.S. Pat. Nos. 2,445,555, Re. 23,297; 4,166,871; and 4,591,512. There is a continuing effort, however, in light of increasing demands placed on polarizer elements and the need for such elements to meet the needs of particular applications, directed toward the development of other methods or treatments for the improvement of properties of light-polarizing materials. There is a particular interest in such methods or treatments which can provide stability against various environmental influences and which can be accomplished simply and efficiently.